1. Technical Field
The present invention relates to an AGC (Automatic Gain Control) circuit built in a television tuner that can receive a digital television signal and an analog television signal.
2. Related Art
A television tuner is known having an AGC circuit that includes an AGC detector circuit for receiving a digital television signal and an AGC detector circuit for receiving an analog television signal and controls a gain of an AGC amplifier circuit. For example, a television tuner 41 shown in FIG. 6 includes an antenna filter 42, an antenna tuning circuit 43, an AGC amplifier circuit 44, a double-tuned circuit 45, and a mixer circuit 46. The AGC amplifier circuit 44 controls a gain so that an output becomes constant in accordance with an input signal level. The television tuner 41 further includes an IF tuning circuit 47 and IF amplifier circuits 48 and 49 that are connected to a rear stage of the mixer circuit 46 in this order. Moreover, the television tuner 41 includes a digital SAW filter 51, a GC amplifier circuit 52, a digital demodulation circuit 53, and an IF AGC circuit 54 that are connected to the rear stage of IF amplifier circuit 49 at a digital receiving side, and includes an analog SAW filter 55 and an analog demodulation circuit 56 that are connected to the IF amplifier circuit 49 at an analog receiving side.
Moreover, the IF amplifier circuit 48 is connected to an AGC detector circuit 58 for receiving a digital television signal to acquire an amplified intermediate frequency signal, and the analog demodulation circuit 56 is connected to an AGC detector circuit 59 for receiving an analog television signal to acquire a demodulated analog signal. The AGC detector circuit 58 for receiving a digital television signal or the AGC detector circuit 59 for receiving an analog television signal applies an AGC voltage to the AGC amplifier circuit 44.
The AGC detector circuit 58 for receiving a digital television signal generates an AGC voltage on the basis of the intermediate frequency signal output from the IF tuning circuit 47, and the AGC detector circuit 59 for receiving an analog television signal generates an AGC voltage on the basis of the demodulated analog signal output from the analog demodulation circuit 56. Each of the AGC detector circuits 58 and 59 is selectively connected to the AGC amplifier circuit 44 via a switching unit 61, and controls the gain of the AGC amplifier circuit 44.
FIG. 7 is a specific configuration diagram of the switching unit 61 and a control circuit thereof.
As shown in FIG. 7, an input port 61a of the switching unit 61 is connected to an AGC signal applying line L3 of the AGC detector circuit 58 for receiving a digital television signal, and an input port 61b of the switching unit 61 is connected to an AGC signal applying line L4 of the AGC detector circuit 59 for receiving an analog television signal. A time constant of the digital-reception AGC signal applying line L3 is set by a capacitor C5 and a resistor R6, and a time constant of the analog-reception AGC signal applying line L4 is set by capacitors C4 and C6 and a resistor R7. An output port 61c of the switching unit 61 is connected to the AGC amplifier circuit 44.
Moreover, the switching unit 61 is connected to a transistor 62, and the switching unit 61 performs a switching operation in accordance with a control of the transistor 62. The switching unit 61 is disconnected from the analog-reception AGC signal applying line L4 when receiving a digital television signal, and is disconnected from the digital-reception AGC signal applying line L3 when receiving an analog television signal. In this manner, because the digital-reception AGC signal applying line L3 and the analog-reception AGC signal applying line L4 have largely different time constants, the switching unit 61 is completely disconnected from one of the AGC signal applying lines to solve a problem due to the disproportionate time constants of the AGC signal applying lines.
See Japanese Unexamined Patent Application Publication No. 2003-189202 [Patent Document 1]
However, because the above-described AGC circuit 57 includes the switching unit 61 and the transistor 62 in order to disconnect the digital-reception AGC signal applying line L3 and the analog-reception AGC signal applying line L4, there is a problem that the circuit scale becomes large and the circuit configuration becomes complicated.